The present invention relates to exhaust gas recirculation effected in internal combustion engines for the purpose of controlling emission of noxious components in exhaust gases from the engine, and, more particularly, to an exhaust gas recirculation control system for controlling the exhaust gas recirculation.
When exhaust gas recirculation is effected in an internal combustion engine, it is important that the amount of exhaust gases recirculated is properly controlled in accordance with operating conditions of the engine, so that the requirement for controlling emission of noxious components is balanced against the requirement for good performance of the engine or the vehicle which mounts the engine.
As an exhaust gas recirculation control system, there has been proposed a system which includes a memory means for electronically retaining target values of the flow of exhaust gases recirculated to provide the optimum exhaust gas recirculation in various operating conditions of the engine expressed by various combinations of conditions selected from rotational speed, amount of intake air and intake manifold vacuum of the engine and a control means which reads out a target value for the flow of recirculating exhaust gases from said memory means in accordance with current values of the parameters selected for expressing the operating condition of the engine, compares the read-out target value with the current flow of recirculating exhaust gases and controls an exhaust gas recirculation control valve so as to accord the flow of recirculating exhaust gases to the read-out target value. In conventional systems of this type, amount of shifting of the valve stem of the exhaust gas recirculation control valve, amount of opening of the butterfly valve of a butterfly type exhaust gas recirculation control valve, intake manifold vacuum of the engine operating under the exhaust gas recirculation, or the exhaust gas recirculation ratio is employed as the target value to be retained in the memory means.
Therefore, in a control system of the aformentioned type, it is required that the same quantity as the target values retained in the memory means should be detected from the currently operating engine, for comparison purposes. Therefore, when the amount of shifting of the valve stem of the exhaust gas recirculation control valve is selected as the quantity of which target values are memorized, a displacement detecting means such as a difference transformer for detecting linear shifting of the valve stem is required. However, since a diaphragm type control valve is generally employed as the exhaust gas recirculation control valve, the stroke of the valve stem is relatively small and requires a high accuracy detecting means. However, the use of a high accuracy detecting means in the automobile causes various problems such as vibration of the valve stem, thermal effects, etc. The butterfly type exhaust gas recirculation control valve has a feature that the flow change rate relative to the rotation of the butterfly valve becomes very large in the small-opening region, and it is difficult to adapt the control system to this particular characteristic. If the intake manifold vacuum of the engine is employed as the quantity of which the target values are memorized, the rotational speed and the amount of intake air must be employed as the parameters for expressing operational condition of the engine. A rotational speed sensor for detecting the rotational speed of the engine over a wide range from low to high speed ranges in high accuracy is currently available in various types. However, a sensor for detecting the amount of intake air in the engine at high accuracy, particularly in the low flow range, is currently not available. When the ratio of exhaust gas recirculation is employed as the quantity of which the target values are memorized, a complicated data processing system is required.